7.1 Solid and Blind Rivet Classifications, Materials & Head Markings

7.1 Solid and Blind Rivet Classifications, Materials & Head Markings

Approved-Data Control

Values and examples explain principles. Current approved maintenance data, product instructions, organisation procedures, and applicable law control actual limits, materials, intervals, methods, and acceptance.

In metallic aircraft construction, riveted joints constitute the primary mechanical fastening method for assembling structural skins, stringers, bulkheads, ribs, and spars. Unlike threaded fasteners that rely on pre-tension and friction between components, rivets join sheet metal assemblies by undergoing controlled permanent plastic deformation. A correctly driven rivet expands radially to fill its hole completely, providing superior shear load transfer and exceptional resistance to cyclic fatigue under flight and cabin pressurization loads. For the certifying aircraft maintenance engineer under EASA Part-66 Module 7 (Maintenance Practices), mastering rivet classifications, material metallurgy, head identification markings, part numbering standards, and blind fastener mechanics is fundamental to maintaining airframe structural integrity.

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Solid Rivet Classifications & Modern Head Geometry

Solid shank rivets are the most widely used fasteners in metal airframes. While dozens of historical head profiles existed during early aviation, modern airframe design and repair have standardized almost exclusively on two primary military standard head configurations:

1. Universal Head (MS20470 / Supersedes AN470)

The MS20470 universal head rivet is the universal workhorse fastener for interior structures and non-flush exterior airframe zones. It combines the high structural strength of the legacy round head (AN430) with the low-profile aerodynamic advantages of the flat head (AN442) and brazier head (AN455/AN456). Modern aircraft specifications have rendered round and brazier head rivets obsolete; standard maintenance practice replaces legacy round, flat, and brazier head rivets with MS20470 universal head fasteners of equivalent diameter and material.

2. 100° Countersunk Flush Head (MS20426 / Supersedes AN426)

The MS20426 countersunk rivet features a flat top surface and a precision 100-degree conical head. It is mandated for exterior airframe skins, flight control surfaces, and engine inlets where laminar airflow is required to minimize parasitic drag. The 100-degree head angle represents the optimum aerospace compromise between maximum bearing surface area and minimum material removal from the outer skin sheet.

Solid Aircraft Rivet Head Profiles:

       MS20470 Universal Head                 MS20426 100° Countersunk Head
              (Exposed)                                   (Flush)

          .-----------------.                         .-----------------.
        /                     \                       \                 /
       |                       |                       \               /
       '---------+   +---------'                        '----+   +----'
                 |   |                                       |   |
                 |   | Shank                                 |   | Shank
                 |   |                                       |   |

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Rivet Metallurgy, Temper Designations & Head Identification Markings

To ensure proper strength and prevent galvanic corrosion, rivets are manufactured from diverse aluminium alloys, nickel alloys, and steels. Because identical-sized rivets look identical once stripped of chemical packaging, aircraft fasteners feature standardized, physical head identification markings (stamped symbols, raised markings, or recessed indentations) forged directly into the manufactured head crown.

Solid Rivet Alloy Classifications

1. 1100 Pure Aluminium (Alloy Code 'A'):

   • Head Marking: Plain, completely smooth head with no markings.
   • Mechanical Properties: Low shear strength (~10,000 psi / 69 MPa); highly ductile.
   • Application: Strictly non-structural utility applications, such as interior trim, electrical grounding brackets, cabin furnishings, and low-pressure air distribution ducts. Never permitted in primary or secondary airframe structures.

2. 2117-T4 Aluminium-Copper Alloy (Alloy Code 'AD'):

   • Head Marking: Single central recessed dimple (dot).
   • Mechanical Properties: Shear strength of ~30,000 psi (207 MPa).
   • Application: Known throughout aviation as the "field rivet". It is the most ubiquitous structural fastener in modern metal airframes. It is delivered in a stable, solution heat-treated and naturally aged temper (T4) and is driven "as received" without any pre-installation thermal treatment.

3. 2017-T4 Aluminium-Copper Alloy (Alloy Code 'D'):

   • Head Marking: Single central raised teat (tit / pip).
   • Mechanical Properties: Shear strength of ~35,000 psi (241 MPa).
   • Application & Handling: Classified as an "icebox rivet". Due to high copper content, 2017-T work-hardens rapidly at room temperature. It must be solution heat-treated in a salt bath or air furnace at ~500°C, immediately quenched in cold water, and placed into a sub-zero freezer maintained at or below -18°C (0°F). Cryogenic storage retards natural age hardening. Once removed from the freezer, the rivet must be driven within 10 to 20 minutes before precipitation hardening renders the shank brittle, causing shear cracking during upsetting.

4. 2024-T4 Aluminium-Copper-Magnesium Alloy (Alloy Code 'DD'):

   • Head Marking: Two parallel raised radial dashes (bars).
   • Mechanical Properties: High shear strength of ~41,000 psi (283 MPa).
   • Application & Handling: The heavy-duty structural "icebox rivet", used in high-shear wing spars, landing gear bulkheads, and primary fuselage frames. Like alloy 'D', 2024-T rivets must be solution heat-treated, water quenched, and stored in sub-zero freezers at -18°C or colder. They must be installed within 10 to 20 minutes of removal from cold storage.

5. 5056 Aluminium-Magnesium Alloy (Alloy Code 'B'):

   • Head Marking: Single raised cross (+).
   • Mechanical Properties: Shear strength of ~28,000 psi (193 MPa).
   • Application: Mandatory for riveting magnesium airframe structures. Magnesium is highly active on the galvanic scale. Standard copper-bearing aluminium rivets (2117, 2017, 2024) create an intense galvanic cell against magnesium, causing catastrophic electrolytic corrosion of the magnesium parent sheet. The 5056 alloy contains ~5% magnesium and zero copper, preventing galvanic corrosion.

6. 7050-T73 Aluminium-Zinc-Magnesium-Copper Alloy (Alloy Code 'E'):

   • Head Marking: Single raised ring (circle).
   • Mechanical Properties: High shear strength (~43,000 psi / 296 MPa) with exceptional stress-corrosion cracking resistance.
   • Application: Driven as received without refrigeration; increasingly replaces 2024-DD icebox rivets in modern manufacturing and heavy repairs.

7. Monel (Alloy Code 'M'):

   • Head Marking: Two recessed dimples.
   • Application: Nickel-copper alloy offering high tensile and shear strength (~49,000 psi) and exceptional heat and corrosion resistance. Installed in firewalls, engine exhaust surrounds, and high-temperature nacelle structures.

8. Corrosion-Resistant Steel (CRES / Stainless Steel - Alloy Code 'F'):

   • Head Marking: Indented letter 'F' or plain recessed markings.
   • Application: Firewalls, engine exhaust cowlings, and structural joints subjected to extreme temperatures exceeding 400°C.

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Military Standard Part Number Decoding Framework

Aircraft rivets are ordered, inventoried, and specified on engineering blueprints using military standard alphanumeric part numbers. Technicians must be able to decode these designations instantly:

           MS20470  AD  4 - 6
              |      |  |   |
              |      |  |   '---> Grip Length in 16ths of an inch (6/16" = 3/8")
              |      |  '-------> Shank Diameter in 32nds of an inch (4/32" = 1/8")
              |      '----------> Alloy Code (AD = 2117-T4, dimple head mark)
              '-----------------> Head Style (MS20470 = Universal Head)

Dimensional Decoding Rules

• Shank Diameter: Specified by the first number following the alloy letter, expressed in 32nds of an inch.
  • Example: 3 = 3/32" (2.38 mm); 4 = 4/32" = 1/8" (3.18 mm); 5 = 5/32" (3.97 mm); 6 = 6/32" = 3/16" (4.76 mm); 8 = 8/32" = 1/4" (6.35 mm).
• Fastener Length: Specified by the dash number, expressed in 16ths of an inch.
  • Example: -4 = 4/16" = 1/4" (6.35 mm); -6 = 6/16" = 3/8" (9.53 mm); -8 = 8/16" = 1/2" (12.7 mm); -12 = 12/16" = 3/4" (19.05 mm).

Critical Head Measurement Difference

• Universal Head (MS20470): Fastener length is measured strictly along the cylindrical shank only, from the flat underside of the head to the shank end.
• Countersunk Head (MS20426): Because the entire conical head sits flush within the sheet metal, fastener length is measured as the overall length, from the flat top surface of the head to the shank end.

Universal Head (MS20470) Measurement:       Countersunk Head (MS20426) Measurement:

         .-------------.                              .-------------. <--- MEASUREMENT
       /                 \                            \             /      STARTS HERE
      |                   |                            \           /
      '---------+   +-----' <--- MEASUREMENT            '---+   +--'
                |   |            STARTS HERE                |   |
                |   |                                       |   |
                |   | LENGTH                                |   | LENGTH
                |   |                                       |   |
                '---' <--------- MEASUREMENT                '---' <------- MEASUREMENT
                                 ENDS HERE                                 ENDS HERE

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Structural Blind Fasteners & Mechanical Lock Mechanics

When structural repairs or assembly must be performed in enclosed structures where the blind side of the work is inaccessible—such as closed box spars, leading edge slats, control surface trailing edges, and tubular assemblies—solid rivets cannot be bucked. In these areas, certified blind fasteners are mandatory.

Open-End Pop Rivets: Prohibited in Aircraft Structure

Commercial utility "pop rivets" feature a hollow tubular sleeve with an internal mandrel held purely by friction. In flight, airframe vibration causes the friction mandrel to shake loose and fall out, leaving an open hole that leaks cabin pressure, admits water, and drastically reduces shear capability. Pop rivets are strictly prohibited on primary and secondary aircraft structures and are limited to non-structural utility brackets.

CherryMAX Mechanically Locked Blind Fasteners (NAS9301 through NAS9312)

The CherryMAX rivet is the premier structural blind fastener across civil and military aviation. It delivers shear and tensile properties equivalent to solid 2117-T and 2024-T rivets through a precision four-piece design:

1. Outer Sleeve: Ductile 5056 aluminium alloy, Monel, or stainless steel sleeve with a universal (NAS9301) or 100° countersunk (NAS9304) head.
2. Pulling Stem (Mandrel): High-strength alloy steel or stainless steel stem featuring a blind-side upset head, a locking collar groove, and a precision break-notch.
3. Driving Anvil: A factory-installed steel ring seated above the manufactured head that engages the pulling head nosepiece.
4. Mechanical Locking Collar: A specialized ductile collar positioned over the stem. During installation, the driving anvil forces this collar downward into a mating counterbore in the rivet head, mechanically locking the stem securely in place before the stem snaps off.

CherryMAX Blind Fastener Cross-Section (Prior to and Post Driving):

       [Prior to Driving]                          [Fully Driven & Locked]

         || Stem (Pulling)                             || Stem breaks flush
        [==] Driving Anvil                             == Locking Collar Driven In
      .---+---. Head                                .---+---. Flush Lock Ring
     /    |    \                                   /    |    \
    |     |     |                                 |     |     |
    +-----+-----+ Outer Sleeve                    +-----+-----+ Blind Upset Formed
    |     |     |                                 |#####|#####|
    |     |     |                                 |#####|#####| Solid Core Retained
    |    / \    |                                 \###########/
    '---+---+---'                                  '---------'
        |   | Mandrel Head                             

Other Aviation Blind Fasteners

• CherryLock (NAS1398/1399): Mechanically locked blind rivet requiring dedicated pulling heads matching each specific rivet diameter. Widely used on legacy transport-category airframes.
• Huck Blind Bolts / Huck-Lok: Heavy-duty, high-strength threaded or swaged blind bolts utilized in high-shear structural joints (such as landing gear bay fittings and wing skin splices).
• Olympic-Lok: Mechanically locked blind fastener utilizing an integral locking ring.

Inspection of Blind Fasteners

Following installation, 100% of blind fasteners must undergo visual and physical inspection:

• Locking Collar Engagement: The locking collar must be completely and evenly seated within the manufactured head counterbore. An unseated or missing locking collar invalidates the fastener's structural certification.
• Stem Break Flushness: The pulling stem must fracture flush with the locking collar within manufacturer tolerances (typically within -0.010" to +0.015" / -0.25 mm to +0.38 mm). A stem that breaks high indicates an under-gripped rivet; a stem that breaks below flush indicates an over-gripped condition.
• Grip Gauge Verification: Technicians must measure total sheet grip using a certified selector hook gauge prior to fastener installation to ensure exact dash-number matching.

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Realistic Maintenance Scenario & Common Exam Traps

Realistic Maintenance Scenario

During an airframe structural inspection on a Bombardier CRJ900, a technician is tasked with replacing five damaged rivets securing an internal wing stringer to the lower wing skin. The SRM specifies MS20470AD5-7 rivets. In the tool crib, the technician discovers the bin for AD5-7 is empty, but a bin labeled MS20470DD5-7 contains plenty of fasteners.

The technician cannot simply substitute DD rivets without adhering to thermal protocols. The DD fastener (2024-T alloy) is an icebox rivet that cannot be driven as received; attempting to drive it at room temperature will cause severe radial cracking of the shop head and overstress the surrounding structure. Furthermore, the technician must not substitute 2117-AD rivets for 2024-DD rivets because 2117-T has a lower ultimate shear strength (30,000 psi vs. 41,000 psi). The engineer correctly requisitions authentic MS20470AD5-7 field rivets from central stores, verifying the recessed dimple on each head prior to driving.

Common Exam Traps

• Trap 1: Confusing rivet head markings. EASA exams routinely test head markings: 'AD' is a recessed dimple, 'D' is a single raised teat, 'DD' is two raised dashes, and 'B' is a raised cross. Remember that 'AD' has the dimple and needs NO heat treatment.
• Trap 2: Length measurement of countersunk vs universal head rivets. Universal head length is measured from the underside of the head to the end of the shank. Countersunk head length is measured overall (top of head to end of shank). Questions asking for the grip or overall length of an MS20426AD4-6 will penalize failing to include head height.
• Trap 3: Using copper-bearing rivets on magnesium structure. Magnesium skins or castings must NEVER be riveted with 2117-AD or 2024-DD fasteners; only 5056 alloy ('B', raised cross) is galvanically compatible.